imx-drm module/dependency changes

- The PRE/PRG drivers added an unwanted DRM dependency to the ipu-v3 driver.
  Remove the dependency by conditionally disabling PRE/PRG support depending
  on CONFIG_DRM.
- Merge the imx-ipuv3-crtc module into the imxdrm module. There is no reason
  anymore for a separation between core drm driver and crtc/plane drivers,
  especially since commit eb8c8880 ("drm/imx: add deferred plane
  disabling"), which added a dependency on imx-ipuv3-crtc to the imxdrm
  module.

* tag 'imx-drm-next-2017-04-04' of git://git.pengutronix.de/git/pza/linux:
  drm/imx: merge imx-drm-core and ipuv3-crtc in one module
  gpu: ipu-v3: don't depend on DRM being enabled